This invention relates to an image sensor, and particularly to a pixel design for such an image sensor. Particularly, but not exclusively, the invention concerns image sensor arrays based on thin film transistor (TFT)-photodiode technology.
Optical image sensors typically comprise a pixel array arranged in rows and columns, with row driving circuitry and column reading circuitry being used to address the array of pixels. Typically, the row and column circuits are provided on a separate substrate to the pixel array, so that interconnections must be provided between each row and the row driver circuit and between each column and the column reading circuit. It is known to introduce multiplexer circuits onto the pixel substrate to enable the number of interconnections to be reduced.
Sensors employing TFT-photodiode pixel circuits have been known for some time, and the driving force behind their development has been, and continues to be, their use in medical diagnostic imaging applications. More recently, interest in optical-based fingerprint sensors has increased. Work has initially been based on diode-diode sensor arrays, but now attention is turning to TFT/photodiode technology because of the lower power consumption, faster read-out and higher multiplexer ratios that can be achieved.
FIG. 1 shows a typical structure of a column multiplexer circuit 10 connected to a TFT/photodiode image sensor array 20. Only a single row in the array 20 is illustrated for simplicity, associated with row conductor 22. Each pixel in the row comprises a TFT 24 and a photodiode 26 connected in series between a common potential 28 and a respective column conductor 30. A signal on the row conductor 22 turns on the TFTs 24 of each pixel in the row, which allows the photocurrent produced in the photodiode to flow to the respective column conductor 30 to be read by a charge sensitive amplifier arrangement 40.
A multiplexer switch 31 in the form of a TFT is connected between each column conductor 30 and the amplifier arrangement 40. The switches 31 are arranged in groups, with each switch 31 in a group being independently controlled by control lines A, B, C, D. These control lines A to D define four multiplexer channels A to D. Each group is provided with an associated charge measurement device 40. However, different groups share the control lines. The arrangement shown provides a 4:1 multiplexing function, and requires four additional control lines A to D.
After the array has been exposed to light, signal charges are stored on the capacitances of the photodiodes. At this point, the array can be read out, and this is done by addressing each row in turn by applying a positive pulse to the appropriate row conductor. In an array without a column multiplexer, each column is connected to its own charge-sensitive amplifier, and when the pixel TFTs 24 are turned on, the signal charge from each pixel flows down the column 30 to the respective charge-sensitive amplifier.
However, in an array with a column multiplexer, the situation is more complicated. Consider the situation when the columns connected to multiplexer channel A are to be read out. This is arranged by turning on the multiplexer TFTs connected to control line A, and ensuring that all other multiplexer TFTs are off. When the row pulse is applied, signal charge from the pixels in the columns associated with multiplexer channel A will flow down the column via the multiplexer switch 31 to the respective charge-sensitive amplifier 40. However, at the same time, the other pixel TFTs 24 will also have been turned on, and signal charge from the photodiodes in those pixels will be transferred to the column capacitance. Hence, the act of reading multiplexer channel A has caused the signal charge from the other pixels to have been lost to the column capacitance.
If a static image is being recorded, for example a fingerprint, the lost charge can be re-created by using multiple exposures. In such a scheme, multiplexer switches A would be turned on, and all rows in the array would be addressed in sequence, thereby reading out charge from all of the pixels connected to columns associated with multiplexer channel A. When this is complete, the array is re-exposed and column multiplexer switches 31 for channel B are turned on. The rows are once again addressed in sequence so that pixel charges from the pixels associated with channel B can be read out. This is repeated for multiplexer channels C and D. While this provides a solution to the charge loss, it suffers from two disadvantages. Firstly, the multiple exposure and read-out lead to a longer image acquisition process which, especially for the fingerprint sensor, is undesirable. The use of multiple exposures is also not appropriate for non-static images, for example in the field of medical diagnostic imaging. Secondly, the data from the array emerges in a column-based sequence. In principle this need not be an issue, but in practice could require the development of bespoke image acquisition and processing software.
To avoid the need for multiple imaging, it is possible for the xe2x80x98lostxe2x80x99 pixel charge to be recovered by transferring the charge from the column capacitances. When reading out multiplexer channels B, C and D by turning on the respective multiplexer switches 31, the column capacitance could be connected to the charge-sensitive amplifier. A number of timing schemes can be devised for such a read-out scheme, but all of them suffer the drawback that the charge is stored on the column capacitance for a period. Further, this period is not the same for all multiplexer channels. The main concern with this type of read-out scheme is the effect of leakage currents from all of the pixels in each column.
A further alternative is to use a complete analogue line store (array of sample and hold circuits) as part of the column multiplexer circuit. With such a circuit in place, the signal charge from the pixels in a given row can be transferred to the line store when the row is addressed. Once this is complete, the charge can be transferred to the charge-sensitive amplifiers via the column multiplexer switches with a timing scheme that is most appropriate for the application. Again, there are several possible implementations, but these basically divide into two types. The first uses a simple switch/capacitor as the sample-and-hold (S/H) circuit, and the second employs a high-gain buffer amplifier as part of the S/H circuit. Both variants have drawbacks: in the first there is charge-sharing between the photodiode capacitor and the column parasitic capacitance; in the second, aside from the added complexity, the issue is the difficulty of implementing a high gain buffer using the same device technology as the pixel TFTs, for example n-channel amorphous silicon technology.
There is a need for an alternative approach which allows column multiplexing to be used with a single exposure image sensor and which is simple to implement. The multiplexing circuitry also needs to be readily implementable using the same technology as the devices of the image sensor pixel, for example n-channel amorphous silicon devices.
U.S. Pat. No. 5,134,489 discloses an image sensor comprising rows and columns of image sensing pixels, each row of pixels being associated with a respective row conductor, and each column of pixels being associated with a respective column conductor, each pixel comprising an image sensing element and a switching device, the switching device enabling a signal of the image sensing element to be provided to the respective column conductor, wherein the switching device is controlled by two inputs, a first input defined by the row conductor, and a second input.
The use of two inputs to the switching device enables an individual pixel within a row, or a group of pixels within a row, to be addressed. In other words, the row address pulse does not result in transfer of charge from the image sensing element of all pixels in the row. The second input is in practice associated with a columns of pixels, so that the two switch inputs can define a unique pixel in the array. A more complicated switching device is required, implemented as two series-connected TFTs in U.S. Pat. No. 5,134,489. This approach also increases the number of control lines for each pixel, which in turn reduces the area of the pixel available for the image sensing element. This reduces the sensitivity of the image sensor device.
According to the invention, two adjacent columns of pixels are associated with each column conductor, the two adjacent columns of pixels being located on opposite sides of the column conductor, the column conductors thereby having a pitch of twice the pitch of the pixel columns, and the two columns of pixels between adjacent column conductors are associated with a respective control line defining the second input.
In this way, a column conductor is shared between two columns of pixels, and the additional control line which provides the second input to the switch is also shared between two columns of pixels. This means that each pixel requires the same number of connections as in a conventional array with the row conductor defining the single switch control line. However, the shared column conductors and shared control lines are staggered, so although the control line addresses two columns of pixels, these are associated with different column conductors, so that the signals may be read separately.
Preferably, therefore, each column conductor is provided with a routing device which is selectively operable to route the signal from the column conductor to a first or a second signal measurement device. The two measurement devices enable the signals from the two column conductors associated with each control line to be read simultaneously.
The shared control line may provide a control signal for the two routing devices of the two columns which are associated with the shared control line, the control signal enabling one of the routing devices to route to the first signal measurement device and the other routing device to route to the second signal measurement device. Thus, the control inputs for the pixels are also used to control the supply of signals to the two measurement devices.
Each routing device may comprise first and second transistors, the first transistor being connected between the column conductor and the first signal measurement device, and the second transistor being connected between the column conductor and the second signal measurement device. These transistors preferably comprise amorphous silicon thin film transistors, and the switching device of each pixel also comprises one or more such transistors, so that the routing devices can readily be integrated with the pixel array.
Each pixel may comprise two series-connected transistors (defining the switching device) and the image sensing element, coupled between the column conductor and a common electrode.
One of the two series-connected transistors of one pixel may also form one of the two series-connected transistors of an adjacent pixel in the same column. Thus, the amount of additional pixel circuitry required can be reduced.